The present invention relates to display of amplitude and phase jitter.
Present day sophistication of telephone line test instruments has brought about a need for measurement of amplitude and phase jitter which are introduced onto a signal while it traverses a transmission medium. An example of a phase jitter meter may be found in U.S. Pat. No. 3,711,773, to Hekimian et al, which patent is expressly incorporated herein by reference. As described in that patent a test signal, having a frequency residing in the pass band of the transmission path under test, is transmitted through that path to an instrument which separates the phase jitter introduced by the transmission path from the test signal. The separated phase jitter signal may be viewed on an oscilloscope (e.g. the "EXT. PHASE JITTER OUTPUT" signal in FIG. 4 of the Hekimian et al patent) or it may be rectified and monitored on a meter (e.g. meter M201 in FIG. 3 of the Hekimian et al patent). Amplitude jitter, which was not a standard measurement at the time the system of the Hekimian et al patent was developed, could also be viewed on an oscilloscope by utilizing the amplitude jitter signal provided at the output of filter 28 in FIG. 2 (corresponding to the output of amplifier 463 in FIG. 5) in the Hekimian et al patent.
An arrangement for simultaneously displaying approximate amplitude and phase jitter on an oscilloscope is described in U.S. Pat. No. 3,906,173, to Bradley. In that patent a co-phase signal is applied to the vertical deflection plates of an oscilloscope while the quadrature signal is applied to the horizontal deflection plates. The resulting display is a centered dot (in the absence of phase and amplitude disturbance) which expands to a vertical line in response to pure amplitude jitter, to a somewhat arcuate horizontal line in response to pure phase jitter, and to a line at some angle between vertical and horizontal in response to simultaneous amplitude and phase jitter. When both amplitude and phase jitter are simultaneously present, the viewer is required to approximate the X and Y axes projections of the angularly displaced trace in order to quantify the phase and amplitude jitter, respectively. This is not always feasible, particularly for short term disturbances (i.e. "hits") of either jitter parameter. Moreover, even when only phase jitter is present, the arcuate nature of the trace requires visual approximation of the X-axis projection for proper quantification.
It is therefore an object of the present invention to provide a display for amplitude and phase jitter which permits each of these parameters to be easily and immediately quantified.
It is another object of the present invention to provide a phase and/or amplitude jitter display which is easily interpreted.
It is still another object of the present invention to provide an improved method of displaying amplitude and phase jitter.